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Hubble's Optics The Vault

Hubble operates on the same principle as reflecting telescopes invented in the 17th century by Newton, Cassegrain, and Gregory. A Cassegrain telescope is a reflecting telescope in which light is reflected from a large primary mirror onto a secondary mirror, which then focuses the light back through a hole in the primary to a point behind the mirror. The science instruments are located directly behind the primary mirror, which, for Hubble, makes instruments easier to replace on-orbit. In Hubble the primary mirrors are exactly configured to eliminate the optical aberrations of ordinary telescopes.

The main mirror in Hubble is about eight feet in diameter. Light enters the telescope and strikes the main mirror. The light is reflected forward to a smaller (12-inch) secondary mirror where the light is reflected again, returning down the telescope through a two-foot hole in the center of the large mirror where the image forms. The science instruments record the images and analyze the incoming light stream.

After launch in 1990, NASA discovered that the large mirror was flawed. The flaw was tiny — about 1/50th the thickness of a piece of paper — but significant enough to distort Hubble's vision. During the First Servicing Mission, astronauts added corrective optics to compensate for the flaw. The optics acted like eyeglasses to correct Hubble's vision.

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